Apparatus for forming pulsed jets of liquid

ABSTRACT

Apparatus for forming pulsed jets of liquid. A movable cylinder cooperates with a nozzle block to define a compressible charge chamber. A piston within the nozzle block is movable between a firing position, in which it completes the nozzle discharge opening from the charge chamber and a loading position, in which it permits passage of gas to the nozzle discharge opening, thereby removing any liquid remaining therein from the preceding liquid jet pulse. To generate a charge, the piston is moved to the loading position, following which the discharge opening is blown dry and charge material is applied to the charge chamber. An actuating piston is actuated to impact on the cylinder compressing the chamber and expelling the charge through the nozzle as a pulsed liquid jet. A floating check valve permits removal of air entrapped within the charge chamber.

United States atent 91 [111 3,905,552

Hall et a1. Sept. 16, 1975 1 APPARATUS FOR FORlVllNG PULSED JETS [57] ABSTRACT OF LIQUID Primary ExaminerM. Henson Wood, Jr. Assistant Examiner-J0hn J. Love Attorney, Agent, or FirmMorton, Bernard, Brown,

Apparatus for forming pulsed jets of liquid. A movable cylinder cooperates with a nozzle block to define a compressible charge chamber. A piston within the nozzle block is movable between a firing position, in which it completes the nozzle discharge opening from the charge chamber and a loading position, in which it permits passage of gas to the nozzle discharge opening, thereby removing any liquid remaining therein from the preceding liquid jet pulse. To generate a charge, the piston is moved to the loading position, following which the discharge opening is blown dry and charge material is applied to the charge chamber. An actuating piston is actuated to impact on the cylinder compressing the chamber and expelling the charge through the nozzle as a pulsed liquid jet. A floating check valve permits removal of air entrapped within the charge chamber.

7 Claims, 6 Drawing Figures Roberts & Sutherland ACTUATO R m Ul I e I uou 0 13 SOURCE 726 PA I ENIEQ 1 51975 3, 9 O5 552 SHCET 1 UF 2 ACTUATOR GAS SOURCE APPARATUS FOR FORMING PULSED JETS OF LIQUID The present invention pertains to an apparatus for forming high pressure pulsed jets of liquid. More particularly, the present invention pertains to a liquid pulsed jet forming apparatus including means for assuring that little, if any, liquid from a previous pulse is remaining in the discharge nozzle at the time a pulsed jet of liquid is expelled through the nozzle, since such remaining liquid might detrimentally affect the pulse generated.

Pulsed jets of liquid are utilized for many purposes, such as fracturing rock, cutting steel or concrete, etc. In typical apparatus for generating such pulsed jets of liquid, liquid enters a reservoir adjacent a pressure cylinder to form a liquid charge. A piston is actuated to contact the pressure cylinder, thereby compressing the reservoir and expelling the liquid charge through a narrow liquid outlet or nozzle at high velocity and pressure. It has been found, however, that following ejection of a jet pulse of liquid, liquid is likely to remain within the nozzle, and this remaining liquid has an adverse effect on the subsequent velocity and/or pressure characteristics of the liquid jet pulse.

The present invention is an apparatus for generating pulsed jets of liquid, including means for assuring that at the time a pulsed liquid jet is generated, little, if any, liquid from a preceding jet remains in the nozzle. In the apparatus in accordance with the present invention, a piston is actuated to move a pressure cylinder, applying pressure to a liquid charge, causing the charge to be expelled through a nozzle at high velocity and high stagnation pressure, for example, a pressure in excess of 100,000 pounds per square inch. Stagnation pressures of 300,000 to 500,000 pounds per square inch and above can be obtained employing the apparatus of the presentinvention. Thus, by a high pressure liquid jet pulse is meant a brief pulse of liquid with a stagnation pressure in excess of 100,000 pounds per square inch. Looking at the apparatus before generating a pulsed liquid jet, a shuttle valve is actuated to isolate the nozzle from the charge chamber and to pass air through the nozzle, thereby blowing any remaining liquid from the nozzle. Liquid charge material is also admitted to the charge chamber for a subsequent liquid jet pulse. A floating check valve is provided to permit escape of any air in the chamber as the charge material enters. To generate a pulsed liquid jet, the shuttle valve is placed in the firing position, which connects the nozzle and charge chamber, and the piston is actuated to very rapidly move the pressure cylinder and compress the charge chamber, thereby expelling the charge out of the nozzle as a liquid jet. The shuttle valve is then again actuated to repeat the nozzle clearing.

The present invention is described in further detail with reference to the drawing in which:

FIG. 1 is a view, partially fragmentary sectional and partially in block form, depicting a preferred embodi ment of apparatus in accordance with the present invention; 1

FIGS. 2A, B and C illustrate schematically in three operational views an actuator for use with the device of FIG. 1; v

FIG. 3 is a fragmentary sectional view illustrating an alternative form of apparatus in accordance with the present invention; and,

block 12 having central forward portion 11 fitted snugly within elongated cylindrical nozzle housing 10. Portion 11 is maintained within nozzle housing 10 by retainer key 16. Extending forwardly from nozzle block 12 and within bore 13 in nozzle housing 10 is elongated forward member 18, which is of a diameter considerably smaller than that of central portion 11. Elongated forward member 18, for example, can be threadedly connected to nozzle block 12. Nozzle block 12 includes a rearwardly extending portion 20, which is preferably coaxial with elongated forward member 18. An axial, tapered bore 22 passes through rearwardly extending portion 20, nozzle block 12, and elongated forward member 18. Although the figure depicts tapered bore 22 as having a uniform taper, this is not essential. Tapered bore 22 could have an areuate or other taper, so long as the cross-sectional area of bore 22 reduces over its length, from a maximum cross-sectional area within rearwardly extending portion 20 to a minimum crosssectional area within elongated forward member 18. If desired, the forward end of forward member 18 can be threaded to receive an extension, like-wise having an axial tapered bore therethrough. Since axial bore, 22 is tapered, a liquid charge traveling through bore 22 from rearwardly extending portion 20 towards elongated forward member 18 has its velocity and pressure increased as it moves. Tapered bore 22 thus serves as an acceleration path or discharge opening for liquid jet pulses traveling through it.

Pressure cylinder 31 is positioned behind rearwardly extending portion 20 and is maintained spaced therefrom by air spring 32 to define compressible chamber 30 therebetween. Pressure chamber 31 is concentrically positioned within cylindrical housing 33 and is longitudinally movable therewithin. A replaceable wear bushing 33' can be used in housing 33 if desired, Retainer nut 36 is threadably attached to housing 33 to hold cylinder 31 and bushing 33' firmly in place. Seals 37 provide a substantially fluid-tight seal between cylinder 31 and housing 33, Pressure seals 38 provide a similar seal between cylinder 31 and portion 20.

Valve bore 60 is provided in central portion of nozzle block 12 at substantially a right angle to bore 22. Shuttle valve 61 includes a piston 62 slidably positioned within sleeve 63 in bore 60. A bore 64 is provided through the piston 62 of shuttle valve 61. Piston 62 is movable between a shuttle valve charge loading position, as shown in FIG. 1, in which bore 64 is not aligned with bore 22' in sleeve 63 and the remainder of tapered bore 22, so that the discharge opening from compressible chamber 30 to the nozzle discharge end of forward member 18 is interrupted, and a shuttle valve firing position, not shown, in which bore 64 is aligned with bore 22 and the remainder of tapered bore 22 to communicate the inlet portion of the tapered bore adjacent chamber 30 with the outlet portion of the tapered bore or the nozzle discharge. Annular plugs 65 and 66 close the two ends of bore 60 through which shuttle valve 61 is inserted to limit the movement of piston 62. A central opening 67 in annular plug 65 permits fluid communication to bore 69 in valve 61 from a gas source through line 70. Gas bore 69 passes through piston 62 to provide communication from this end of shuttle valve 61 to the outlet portion of tapered bore 22 when shuttle valve 61 is in its charge loading position. Recess 68 is provided in piston 62 in a position such that with shuttle valve 61 in its charge loading position, recess 68 is adjacent the portion of tapered bore 22, communicating with the chamber 30. FIG. 4 illustrates button seals 74 and 74 in recess 68 and 68. A bore 71 in piston 62 connects bore 71 in the button seal 74 and bore 71 in button seal 74. In the shuttle valve charge loading position, conduit 72, through nozzle block 12, provides a liquid inlet path from liquid source 73 through conduit 72, bore 71', bore 71", bore 71 and bore 22 to compressible chamber 30. Metal button seals 74 and 74 are biased by springs 75 and 75 to seal, respectively, on bushing 63 and piston 62. Thus, seals 74 and 74 prevent escape of liquid from recess 68 to other areas of bore 60.

Movement of piston 62 is controlled by the gas pressure from the gas source at bore 67 and plunger 76 which acts against piston 62. Plunger 76 slides within bore 77 of mounting 78 in plug 66 in response to hydraulic pressure at the bore,. as more fully described hereinafter. Seals 79 are provided to keep the hydraulic fluid from escaping into bore 60. Retainer 78 holds mounting 78 in place. Piston 46 is positioned behind pressure cylinder 31 and is surrounded by jacket 48, within which it fits snugly, but slidingly. Preferably, a replaceable wear bushing (not shown) is provided on the interior wall ofjacket 48. Outer sleeve 51 ofjacket 48 fits over housing 33, which has an elongated recess 52 adjacent sleeve 51. Retainer S4 is located within rccess 52 to maintain housing 33 and sleeve 51 together while permitting longitudinal movement therebetween from the position illustrated in FIG. 1, which is the firing position of the apparatus with housing 33 withdrawn into sleeve 51 until the forward end of recess 52 contacts retainer 54 to a non-firing position in which housing is extended from sleeve 51 with the rearward end of recess 52 contacting retainer 54. In this non-firing position with retainer 54 at the rearward end of recess 52, pressure cylinder 31 is sufficiently distant from piston 46 to prevent contact therebetween and thus to prevent accidental discharge of the apparatus. A spacer can be provided, if desired, between sleeve 51 and housing 33 forward of recess 52, and retainer nut 59 engages jacket 51 to maintain the components together.

Floating valve 85 is provided to allow venting of air from compressible chamber 30. Bore 80 provides fluid communication through pressure cylinder 31 from valve chamber 81 to compressible chamber 30. Bore 82 provides fluid communication through pressure cylinder 31 from valve chamber 81 to axial slot 84. Bore 86 provides fluid communication from axial slot 84 through housing 33 to the exterior of pulsedjet forming device 9.

Valve body 87 has a diameter slightly less than that of valve chamber 81 and includes a plurality of fins 90 extending outwardly from valve body 87 to contact the wall of valve chamber 81. Extension 92 extends from valve body 87 and terminates in a tapered end 94. The junction of bore 82 and valve housing 81 is tapered to substantially match the tapered portion of end 94. Spring 96 surrounds forward extension 92 and urges tapered end 94 away from bore 82.

Piston 46 can be actuated by any suitable means, depicted diagrammatically as actuator 58. Such means might involve release of high pressure fluid, detonation of an explosive charge, release of a cocked spring, etc.,

to cause piston 46 to impact against pressure cylinder 31, moving the pressure cylinder and compressing chamber 30 to expel liquid from within chamber 30 through tapered bore 22 as a pulsed liquid jet. One such commercially available means is shown in FIGS. 2A, B and C, which diagrammatically illustrate its oper ation. After each impact, hydraulic fluid in line 104 lifts piston 46, expelling the fluid above it through line 106. Piston 46 moves up, closing its upper opening against seal face 108. The system is then compressed by fluid continuing to enter at 104. This fluid now enters the charging chamber 110, pushing piston 112 down and compressing the gas, e.g., nitrogen, in chamber 114. The forces at the seal face and in chambers 110 and 114 are balanced such that the seal is broken at a desired pressure, which triggers the instant release of the stored energy. Piston 112 moves upward and piston 46 is forced downward with explosive force to strike pressure cylinder 31. Operation of pulsed liquid jet forming device 9 and, particularly, shuttle valve 61, is under the control of the actuator. In general, a characteristic of the actuator is selected and the valve responds thereto. For example, if the actuator is hydraulically operated, as in FIG. 2, the pressure of the hydraulic fluid is a suitable characteristic. When this pressure reaches a point just before it actuates to strike piston 46, the pressure is used to start the operational sequence of device 9, which is then automatic.

For operation of device 9, the gas source applies gas, e.g., compressed air, to bore 67 and air spring 32 and liquid source provides liquid, e.g., water, to line 72. The forward end of cylinder 18 preferably contacts the workpiece at which the pulsed liquid jet is directed to retain pulsed liquid jet forming device 9 in its firing position, with housing 33 withdrawn into sleeve 51. Assuming that a pulsed jet has just been fired, there is no pressure in the actuator or, therefore, applied to plunger 76. Accordingly, the gas pressure in bore 67 moves piston 62 and places shuttle valve 61 in its charge loading position. The gas from source 68 in the charge loading position then passes through bore 69 to tapered bore 22 to force from bore 22 any liquid re maining from the preceding pulsed jet. In this position, liquid source 73 is connected through line 72 and bores 71, 71 and 71", and the inlet portion of bore 22 to compressible chamber 30 and liquid fills the chamber. The strength of spring 96 in the floating check valve is selected so that the force of liquid entering into chamber 30 causes the air within chamber 30 to push valve body 87 and out bores 82 and 86 without closing bore 82.

When it is desired to create a liquid jet, actuator 58 is energized. At a pre-selected hydraulic pressure in the actuator, and prior to breaking of the seal at face 108, the pressure applied against plunger 76 through line 116 overcomes the gas pressure applied against piston 62 and through bore 67 and moves piston 62, so that shuttle valve 61 is in the firing position in which bore 64 completes tapered bore 22. Actuator 58 continues its operational sequence to cause piston 46 to impact against pressure cylinder 31. As a result, pressure cylinder 31 is impelled toward nozzle block 12, compressing chamber 30 and forcing the liquid therein out tapered bore 22 as a pulsed liquid jet. During this movement of pressure cylinder 31, the rapid buildup of pressure in chamber 30 moves valve body 87 of check valve 85 against the bias of spring 96 so that tapered end 94 contacts the tapered inlet bore 82, preventing passage of liquid thereinto. The liquid charge material is expelled from elongated forward member 18 as a pulsed jet of liquid, generally having a pressure of at least 100,000 psi, and, preferably, a pressure in the range of from about 300,000 p.s.i., or higher. Following generation of the pulsed jet, air spring 32 returns pressure cylinder 31 to its rearward position. Spring 32 can be connected to gas source 68 if desired. Using the device of FIG. 1, jets having pressures of at least 100,000 p.s.i., and higher, have been generated and used to break rock and cut through steel.

FIG. 3 illustrates an alternative embodiment of apparatus in accordance with the present invention in which line 125 from liquid source 73 communicates through check valve 125' to bore 71, which passes through nozzle block 12 to bore 22 and compressible chamber 30. Thus, the liquid charge material is applied directly to chamber 30 and not through the shuttle valve. Cheek valve 125' prevents flow from chamber 30 back through line 71 during generation of a pulsed jet. As a further alternative, the liquid inlet path from line 72 to compressible chamber 30 could be provided through pressure cylinder 31.

FIG. 3 also illustrates a modified shuttle valve 61' and illustrates the valve in firing position with bore 69' connected with tapered bore 22 and with bore 64' blocked by nozzle block 12. In this modification, gas pressure from source 68' is used to move piston 62 in both directions. A three-way valve 130 is operated by the hydraulic pressure of actuator 58 to pass the gas alternatively to line 132 or 134 and thereby move the piston 62'. Piston 62 would normally be maintained so the valve 61' is in the non-firing position. The inclusion of gas source 68' to provide gas to force liquid from tapered bore 22 makes convenient the use of that same gas source to control the position of valve 61.

Although the present invention has been described with reference to preferred embodiments, numerous modifications and rearrangements could be made and still the result would be within the scope of the invention.

What is claimed is:

1. Apparatus for forming pulsed jets of liquid comprising:

a nozzle block having a discharge opening therethrough and a valve bore therein transverse said nozzle block discharge opening to divide said nozzle block discharge opening into an inlet portion and an outlet portion;

a pressure cylinder cooperating with said nozzle block to define a compressible chamber in fluid communication with said nozzle block discharge opening inlet portion;

a valve body slidably mounted within said nozzle block valve bore and having therethrough a discharge opening, adapted to be positioned in line with said nozzle block discharge opening, and a gas opening, said gas opening having a first end adapted to be interconnected in fluid communication with said nozzle block discharge opening outlet portion and having a second end adapted to be connected to a source of gas; said valve body slidable within said valve bore between a first valve position, in which said valve body discharge opening is aligned with said nozzle block discharge opening to provide a discharge path for said compressible chamber and said gas opening is not interconnected with said nozzle block discharge opening outlet portion, and a second valve position, in which said valve body discharge opening is not aligned with said nozzle block discharge opening, thereby interrupting the compressible chamber discharge path, while said valve body gas opening first end is in fluid communication with said nozzle block discharge opening outlet portion;

liquid supply means for supplying liquid charge material to said chamber;

valve control means for controlling movement of said valve body between the first valve position and the second valve position;

gas source means connected to said nozzle block gas opening second end for supplying gas thereto;

bias means for normally urging said pressure cylinder from said nozzle block to enlarge said compressible chamber;

impelling means for rapidly acting on said pressure cylinder to compress said compressible chamber against the urging of said bias means and to expel liquid charge material from said compressible chamber out said discharge opening as a pulsed liquid jet, including means for selectively controlling operation of said valve control means to selectively cause:

a. the positioning of said valve body in the second valve position, when the impelling means is not acting on said pressure cylinder, to permit gas to pass from said gas source means through said valve body gas opening and out said nozzle block discharge opening outlet portion to remove liquid therefrom; and

b. the positioning of said valve body in the first valve position prior to actual expulsion of the liquid charge from the chamber, upon activation of said impelling means to compress said compressible chamber.

2. Apparatus as claimed in claim I wherein said impelling means is hydraulically actuated and in which said valve control means comprises means acting upon said valve body in response to the hydraulic pressure in said impelling means to move the valve body to said first valve position, and in which said nozzle block gas opening second end is positioned to pass gas from said gas source means to act against said valve body in 0pposition to said hydraulic pressure in a manner so as to move said valve body to the second valve position against the bias of said hydraulic pressure, the hydraulic pressure at a pre-selected time overcoming the pressure of the gas to move the valve body.

3. Apparatus claimed in claim 1 wherein said valve control means comprises first and second gas inlets to said nozzle block, means coupling said first and second gas inlets to said gas source, and means for selectively connecting said gas source to one of two connecting means comprising first connecting means for passing gas through said nozzle block first gas inlet to cause movement of said valve body from the first valve position to the second valve position and second connecting means for passing gas through said nozzle block second gas inlet to cause movement of said valve body from the second valve position to the first valve position.

4. Apparatus as claimed in claim 1 in which said liquid supply means includes a liquid passageway in said nozzle block adapted to be connected to a source of liquid charge material and a recess in said valve body positioned to provide liquid communication between said nozzle block liquid passageway and said nozzle block discharge opening inlet portion when said valve body is in the second valve position.

5. Apparatus as claimed in claim 1 in which said liquid supply means includes a liquid supply conduit in said nozzle block adapted to connect said compressible chamber with a source of liquid charge material and check valve means in said liquid supply conduit for closing said conduit upon an increase in pressure in said compressible chamber.

6. Apparatus as defined in claim 3 wherein said means for selectively connecting said gas source comprises a hydraulically actuated three-way valve and wherein said impelling means is hydraulically actuated and including means for switching said three-way valve between said first and second connecting means in response to the hydraulic pressure in the impelling means.

7. Apparatus as claimed in claim 1 in which one of said nozzle block and said pressure cylinder has a vent passageway therethrough communicating said compressible chamber with the exterior of said apparatus and in which said apparatus further comprises floating check valve means in said vent passageway normally permitting fluid flow through said vent passageway, so that fluid from said compressible chamber is permitted to pass said floating check valve means and be vented from said compressible chamber, said check valve means operating upon an increase in pressure in said compressible chamber to close said vent passageway.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECT-ION PATENT NO. 3,905,552

DATED September 16, 1975 |NVENTOR(S) J. MICHAEL HALL and LOUIS L. CLIPP It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 57, "drawing" should read -drawings;

Column 2, line 24, "like-wise" should read likewise-;

Column 2, line 35, "chamber" should read cylinder;

Column 2, line 66, "provide communication" should read provide fluid communication-;

Column 3, line 4, "with the chamber" should read with chamber;

Column 3, line 5, "recess" should read -recesses;

Column '4, line 6, "2A, B and C" should read 2a, b and c;

Column 4, line 49, after "push" insert -past;

Column 4, line 57, "62 and through" should read 62 through-;

Column 5, line 1, after "inlet" insert -of Column 5, line 6, after "300,000 p.s.i.," insert to about 500,00 p.S.i.-.

Signed and Scaled this twenty-third D ay Of December I 9 75 [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Commissioner ufPatents and Trademarks A Nesting Officer 

1. Apparatus for forming pulsed jets of liquid comprising: a nozzle block having a discharge opening therethrough and a valve bore therein transverse said nozzle block discharge opening to divide said nozzle block discharge opening into an inlet portion and an outlet portion; a pressure cylinder cooperating with said nozzle block to define a compressible chamber in fluid communication with said nozzle block discharge opening inlet portion; a valve body slidably mounted within said nozzle block valve bore and having therethrough a discharge opening, adapted to be positioned in line with said nozzle block discharge opening, and a gas opening, said gas opening having a first end adapted to be interconnected in fluid communication with said nozzle block discharge opening outlet portion and having a second end adapted to be connected to a source of gas; said valve body slidable within said valve bore between a first valve position, in which said valve body discharge opening is aligned with said nozzle block discharge opening to provide a discharge path for said compressible chamber and said gas opening is not interconnected with said nozzle block discharge opening outlet portion, and a second valve position, in which said valve body discharge opening is not aligned with said nozzle block discharge opening, thereby interrupting the compressible chamber discharge path, while said valve body gas opening first end is in fluid communication with said nozzle block discharge opening outlet portion; liquid supply means for supplying liquid charge material to said chamber; valve control means for controlling movement of said valve body between the first valve position and the second valve position; gas source means connected to said nozzle block gas opening second end for supplying gas thereto; bias means for normally urging said pressure cylinder from said nozzle block to enlarge said compressible chamber; impelling means for rapidly acting on said pressure cylinder to compress said compressible chamber against the urging of said bias means and to expel liquid charge material from said compressible chamber out said discharge opeNing as a pulsed liquid jet, including means for selectively controlling operation of said valve control means to selectively cause: a. the positioning of said valve body in the second valve position, when the impelling means is not acting on said pressure cylinder, to permit gas to pass from said gas source means through said valve body gas opening and out said nozzle block discharge opening outlet portion to remove liquid therefrom; and b. the positioning of said valve body in the first valve position prior to actual expulsion of the liquid charge from the chamber, upon activation of said impelling means to compress said compressible chamber.
 2. Apparatus as claimed in claim 1 wherein said impelling means is hydraulically actuated and in which said valve control means comprises means acting upon said valve body in response to the hydraulic pressure in said impelling means to move the valve body to said first valve position, and in which said nozzle block gas opening second end is positioned to pass gas from said gas source means to act against said valve body in opposition to said hydraulic pressure in a manner so as to move said valve body to the second valve position against the bias of said hydraulic pressure, the hydraulic pressure at a pre-selected time overcoming the pressure of the gas to move the valve body.
 3. Apparatus as claimed in claim 1 wherein said valve control means comprises first and second gas inlets to said nozzle block, means coupling said first and second gas inlets to said gas source, and means for selectively connecting said gas source to one of two connecting means comprising first connecting means for passing gas through said nozzle block first gas inlet to cause movement of said valve body from the first valve position to the second valve position and second connecting means for passing gas through said nozzle block second gas inlet to cause movement of said valve body from the second valve position to the first valve position.
 4. Apparatus as claimed in claim 1 in which said liquid supply means includes a liquid passageway in said nozzle block adapted to be connected to a source of liquid charge material and a recess in said valve body positioned to provide liquid communication between said nozzle block liquid passageway and said nozzle block discharge opening inlet portion when said valve body is in the second valve position.
 5. Apparatus as claimed in claim 1 in which said liquid supply means includes a liquid supply conduit in said nozzle block adapted to connect said compressible chamber with a source of liquid charge material and check valve means in said liquid supply conduit for closing said conduit upon an increase in pressure in said compressible chamber.
 6. Apparatus as defined in claim 3 wherein said means for selectively connecting said gas source comprises a hydraulically actuated three-way valve and wherein said impelling means is hydraulically actuated and including means for switching said three-way valve between said first and second connecting means in response to the hydraulic pressure in the impelling means.
 7. Apparatus as claimed in claim 1 in which one of said nozzle block and said pressure cylinder has a vent passageway therethrough communicating said compressible chamber with the exterior of said apparatus and in which said apparatus further comprises floating check valve means in said vent passageway normally permitting fluid flow through said vent passageway, so that fluid from said compressible chamber is permitted to pass said floating check valve means and be vented from said compressible chamber, said check valve means operating upon an increase in pressure in said compressible chamber to close said vent passageway. 